Presently, there are significant uncertainties when it comes to predicting the future demand for broadband telecommunications services such as high definition video and interactive data communications. This uncertainty in the future demand for broadband telecommunications services has a significant impact on the design of public telephone networks. First, to satisfy the unknown growth pattern in future service demands, it is desirable to have a robust network design that can be easily modified in response to changes in demand for particular telecommunications services. Second, the network must be able to handle vastly different types of traffic ranging from low speed data and voice to full motion video. Third, depending on the demand for wideband services, a network design must be capable of providing a migration strategy from existing copper wires and circuit transmission and switching facilities to optical fibers and the succeeding generations of high speed packet transmission and switching facilities, which packet facilities are used in connection with the delivery of wideband telecommunications services. These three criteria determine the selection of the three major components of a network design: network topology, transmission systems and switching systems. Here, the concern is primarily with transmission systems and transmission techniques which meet the foregoing criteria.
Two important types of commercially used transmission systems are circuit systems and packet systems. Typically, circuit systems utilize time division multiplexing (TDM) as a transmission technique. When TDM is used, each data stream comprises frames which are subdivided into slots. Corresponding slots in each frame are allocated to specific connections. For example, the first slot in each frame is allocated to one specific connection and the second slot in each frame is allocated to a second connection, etc. Each frame also includes a field which contains transmission overhead information including frame synchronization words and control words. This traditional circuit transmission format can be extended to multiple bit rate services by allocating multiple slots in each frame to high bandwidth services. In such circuit transmission systems, a combination of space division switching and time division switching is utilized at the network switches to swap time slots between various bit streams so that connections to and between specific subscribers are established.
Historically, the first digital circuit transmission systems were introduced during the 1960's . These first digital circuit transmission systems were introduced in inter-office trunking applications to carry 24 voice channels by a single 1.544 Mb/sec digital stream. This is known as the DS-1 signal. Subsequently, the wide deployment of digital channel banks in the public telephone network required the multiplexing of several DS-1 signals into a higher speed bit stream to efficiently utilize available transmission links. As the network grew further, continuing efforts to effectively multiplex tributaries having different bit rates into a common bit stream resulted in the well-known hierarchical multiplexing plan comprising the DS-1 (1.544 Mb/sec), DS-1C (3.152 Mb/sec), DS-2 (6.312 Mbit/sec), DS-3 (44.736 Mb/sec) and DS4 274.176 Mb/sec signals.
Conventional circuit transmission systems suffer from a number of shortcomings. Perhaps the most important problem is the multiplexing hierarchy itself. An important result of the hierarchy is an inherent lack of flexibility. Since the network can only transmit the set of signals in the hierarchy, every telecommunications service has to meet the stringent interface requirement of given hierarchical signal bit rates, instead of the particular service being able to transmit at its own natural bit rate. Therefore, the packet mode of transmission which is inherently bit rate flexible is favored for future broadband networks which are to be adapted to deliver enhanced telecommunication services such as high definition video and interactive data communications.
In contrast with circuit transmission systems which transmit data in frames subdivided into slots, packet transmission systems transmit data in discrete blocks or packets, with each packet having an address header at the front thereof. At the network switches, packets are routed from a specific input line to a specific output line, based on address information contained in the packet header. In this way data packets can be routed from a particular subscriber location, through a telecommunications network, to another subscriber location. Packet transmission techniques and especially fast packet transmission techniques (see e.g., R. W. Muise et al., "Experiments in Wideband Packet Technology", Proc 1986 International Zurich Seminar on Digital Communications, pp. 136-138) are inherently bandwidth flexible (i.e. the number of packets generated by a given service per unit time is flexible) and thus are suitable for wideband enhanced communications services. Accordingly, it is desirable to introduce packet transmission technology into the public telephone network, which up to now is based primarily on circuit transmission technology.
The commonly-held view as to how to introduce packet technology into the public network is to deploy a packet overlay network because the existing network is optimized for circuit transmission and is therefore incompatible with packet transmission techniques. Accordingly, many deployment strategies recommend constructing an overlay packet network for a set of wideband services and hope that the migration of new services to the packet overlay network will allow the existing circuit transmission network to be phased out slowly. The main advantage of a packet overlay network is the quick realization of an end-to-end network for new services. However, the approach requires a large initial capital investment and increases operational cost by requiring the management of multiple separate networks.
As described in the above noted related applications, it is desired to provide an alternate approach for introducing packet transmission technology the public telephone network, which approach requires the replacement of existing transmission components but not the implementation of an entirely new network. Thus, is desired to provide a digital data transmission system capable of handling both existing hierarchical circuit traffic and packet traffic.
It should be noted that recent advances in network switch designs have blurred the distinction between packet networks and circuit networks. A typical switch for use in a telecommunications network has three major components: control processor, switch interfaces, and interconnection network. The control processor handles call set-up and tear-down, maintenance and administrative functions. The switch interfaces convert transmission formats (i.e., the format data has when transmitted between switching nodes) to switch formats (i.e., the format data has when processed within switching nodes). The interconnection network routes information blocks from specific input lines to specific output lines of the switch. For the existing digital circuit systems used in the public telephone network, the information in a specific time slot on an incoming line is transferred, via the switch, to a specific time slot on an outgoing line. Thus, the interconnection network serves as a cross-connect for the incoming signals on a slot-by-slot basis.
It has recently been shown (see e.g., Day-Giacopelli-Huang-Wu U.S. patent application Ser. No. 021,664 entitled Time Division Circuit Switch, filed on Mar. 4, 1987 and assigned to the assignee hereof now U.S. Pat. No. 4,782,478, issued Nov. 1, 1988) switch for use in a circuit network can be built using a self- routing packet interconnection network. An example of such a self-routing packet network is the Batcher-banyan network. Based on the address headers associated with fixed sized packets, the Batcher-banyan network routes a plurality of packets in parallel to specific destination addresses (i.e., specific output lines) without internal collisions. Thus, to mimic the operation of the conventional time-space-time switches used in circuit networks, switch interfaces are provided which perform the time slot interchange function and which are able to insert headers in front of circuit slots to convert such slots into packets for routing through the self-routing interconnection network and able to remove headers from packets leaving the self-routing interconnection network to reconvert packets back into conventional circuit time-slot format.
In addition to circuit and packet transmission, another mode of digital transmission is known as Asynchronous Time Division Multiplexing (ATDM). See e.g., W. W. Chu "A Study of Asynchronous Time Division Multiplexing for Time Sharing Computer Systems" Proc AFIPS Vol 35, pp. 669-678, 1969 and A. Thomas et al. "Asynchronous Time Division Techniques: An Experimental Packet Network Integrating Video Communication" Proc International Switching Symposium, May 1984. ATDM is used in connection with continuous and bursty data traffic. ATDM uses channel identifiers with actual data to allow on-demand multiplexing of data from subscriber terminals with low channel utilization. The channel identifiers and associated data form time slots. However, ATDM is bit rate flexible since the appearance of packets can be asynchronous. Slot timing is obtained from a special synchronization pattern which is inserted into unused time slots. Since the synchronization pattern appears only in unused time slots, ATDM cannot be used to carry existing high speed hierarchical signals wherein the loading is close to one hundred percent.
In short, the situation is that the present public telephone network utilizes circuit transmission technology and the associated time division multiplexing transmission techniques, while future broadband services, the demand for which is presently uncertain, are best offered using packet transmission technology. It is therefore an object of the invention to provide a transmission system which is capable of integrating present circuit traffic with future packet traffic so as to provide a flexible migration strategy from the existing copper wire based circuit network to succeeding generations of high bandwidth packet transmission networks. It is a further object of the invention to provide a circuit known as a framer circuit, which circuit is important for the implementation of specific multiplexers and demultiplexers suitable for use in a network utilizing the inventive transmission systems.